As a result of the increasing cost of wood, there exists a demand for alternatives to solid wood construction products. Accordingly, the use of plastic extrusion profiles as substitutes for wood products in the areas of door, wall and window mouldings and picture frames has increased significantly in recent years. Plastic extrusion processes of materials such as polyurethane, polyvinylchloride (PVC) and polystyrene, for example, especially when foamed can provide a substitute for a wood picture frame or door, wall or window moulding.
Sometimes the plastics extrusion may also need to be decorated with ornamental features to enhance the beauty and desirability of the ultimate product. Various types of designs and patterns, such as for example flowers, leaves or scrolls, are used to cover and decorate the plastics extrusion profiles. Door panels for example may be provided with a decoration in relief.
Different techniques exist to apply a decoration to panels or extruded profiles. Existing materials and techniques of forming substantially rigid plastic articles, however, do not allow for efficient economical manufacture of products that have a decoration in relief.
One method consists of injection moulding, at low pressure, a composition of a certain material, preferably a foaming material. The mould presents the desired decoration pattern, and the injected material expands inside the mould. This method does not allow for low cost, large scale (high production rate) manufacture of sizeable ornamented plastic articles.
It is known from EP-0 731 759 to manufacture decorative panels with a decoration in relief by first heating up substantially homogeneously to a temperature at which the panel becomes soft. That way, all residual tensions previously present in the panel as a result of its extrusion are completely relaxed and a homogeneous, plastic mass is obtained. Then, with the help of a press and a mould, which is preferably brought to the same temperature as the soft foam panel, a decoration is applied. After pressing, the mould is left for a time in contact with the foam panel, while the latter is allowed to cool until it is substantially hardened again, after which the mould and the foam panel are separated from each other. This method has the disadvantage that substantially homogeneously heating a panel is very time-consuming. Depending on the material and the thickness of the panel, at least 20 minutes is needed to complete the whole operation.
A method of forming an ornamental or decorative enhancement integral with an extrusion profile is described in EP-0 723 845. The decorative enhancement is formed integrally with the profile by use of an embossment technique on the profile before the extrusion material sets and hardens. Therefore, a heated embossing wheel is driven over a portion of the profile to be decorated. The heat retained in the extrusion profile, as well as the heat in the embossing wheel, allows the formation of a decorative pattern in the profile. The decorative pattern is allowed to cure and harden along with the remainder of the profile. This method needs be carried out at a moment in time shortly after extrusion of the profile, when the profile is still hot. It cannot be done on profiles previously extruded and taken out of stock. An embossing roller is not able to emboss to the same depth as a moulding technique.
EP-0 852 997 describes a method of forming ornamented plastic articles, which comprises firstly extruding plastics material, and then re-moulding it in a hot stamp press. It has been found, according to this patent application, that by controlling the temperature of the press not only at the die above but also at the platen below the extruded profile, any tendency of the extruded profile to bow upwardly under the pressing action may be counteracted. Therefore, the ornamented plastic articles in principle should come out of the press straight. However, if environmental or materials parameters change, the conditions in the press need to be altered to restore product without camber. The die and plate of the press are large massive parts which have a high thermal capacity. Their temperature cannot be changed very quickly. Therefore, upon changing environmental parameters, a non-negligible amount of ornamented plastics profiles will be generated which have camber or production must be stopped until the press has reached equilibrium again. EP-0 852 997 mentions that the system cannot prevent surface damage and defects in the pressed product. No solution to this problem is proposed other than to accept the damage as an aesthetic effect.